Crupper

spelled crouper[2]) is a piece of tack used on horses and other equids to keep a saddle, harness or other equipment from sliding forward.

A crupper is used to keep the equipment placed on a horse's back from slipping forward.

A crupper needs to be snug enough to keep the saddle or harness in place, but not so tight that the horse is irritated or the skin of the tail is damaged.

Cruppers are adjusted to engage only when needed; pressure is not meant to be constantly applied.

[5] If used in an improper manner for too long a period of time, the horse may even become disabled in the hindquarters.

[7] For driving, a crupper is used to prevent the parts of a harness that fit around the barrel of the horse (the girth, bellyband, back band and saddle[4]) from slipping forward.

[6][17] Cruppers continue to be part of the ceremonial tack for some cavalry regiments and tent pegging teams.

Crupper goes under the top of the tail and helps to stabilize harness
Crupper attached to the back of a saddle
Antique Japanese cruppers designed for saddles
The crupper goes directly under the tailhead
Traditional attire and tack of a buttero , utilizing a crupper.